The protagonist is perhaps a doctor. Or maybe a lawyer, teacher...
But most probably a medic, since his services were needed immediately.

So he, an outsider, was called into some remote town to treat a little girl.
Insiders were not happy but saw it as required risk.

Alternatively, perhaps a girl was brought to him, and after treating her few days later he wants to check if her health is going better and visits her hometown.

The guy starts noting the weird girl's behaviour, like she had memory gaps or split personality. He thinks, looking at items around, there are three girls with radically different interests and tastes live in the house, but sees there is a single child's bed there. Step by step, he deduces that EVERYONE in the town has three personalities co-inhabiting their bodies. They stick together in this remote places because they don't want to be seen as a potentially dangerous "alien race" of sorts.

In the end he moves away from the town as that girl's elder sister, who studied faraway to become a town medic, comes back from a city.

When did you read this? In English? How long was the book? Do you remember any of the cover art? We have a list of questions at scifi.stackexchange.com/tags/story-identification/info to help clarify. If you could go through those and see if there's more information you could provide, we'd be much obliged.
– FuzzyBootsJul 25 '17 at 20:59

It was most probably American Sci-Fi of last half of XX century. I read translated to Russian in electronic form. It should be a novel, perhaps about 1-3 dozen pages.
– AriochJul 25 '17 at 21:01

So 12-36 pages? More of a short story?
– FuzzyBootsJul 25 '17 at 21:02

yep, perhaps. I am not native speaker, so would not feel the border between those fine.
– AriochJul 25 '17 at 21:21